In recent years, a water absorbing agent aiming at absorption of body fluids has been widely utilized in sanitary goods such as a disposable diaper, a sanitary napkin, an incontinence pad, and the like. As this water absorbing agent, for example, cross-linked polymers of partially neutralized polyacrylic acid and the like have been known, which have been generally used in particulate forms although those in the form of sheet-like, fibrous, film-like or the like have been also known.
Characteristics expected for this water absorbing agent include water-absorption properties, i.e., high absorption capacity without load and high absorption capacity under a load, of course. However, problems of sanitary aspects and odor must be also considered because it may be used in sanitary goods such as disposable diapers. In connection with causes that affect the problems of sanitary aspects and odor, they may result from the raw materials of the water absorbing agent. Among them, one known cause may be attributed to an unreacted monomer that remains in the water absorbing agent in a slight amount. Therefore, the amount of residual monomer being low in the water absorbing agent has been desired.
As techniques for reducing the amount of residual monomer in a water absorbent resin, (1) a process in which amount of a radical polymerization initiator added to a monomer liquid is increased by adding two times or more in divided fractions (JP-B No. S63-7203), (2) a process in which a radical polymerization initiator is added to a hydrogel polymer during or following polymerization (JP-A No. 2004-517179), and (3) a process in which a reductive substance is added to a hydrogel polymer prior to or during drying (JP-B No. H7-98847) were disclosed. Moreover, (4) a process in which a reductive substance is mixed in combination when a water absorbent resin is mixed with an aqueous surface crosslinking agent solution (JP-A No. H04-106108), and (5) a process in which a reductive substance is mixed when a water absorbent resin after completing a surface crosslinking treatment is mixed with a mixing activator such as a surfactant and water insoluble fine particles (pamphlet of International Publication No. 91/03497), and the like were also disclosed.
However, when the amount of residual monomer is reduced using the technique described in the above processes (1) and (2), the resulting water absorbent resin may be colored. Thus, the water absorbent resin has come to be seen through a top sheet of the absorbing article, leading to a problem of deterioration of the commercial value. Furthermore, because various severe conditions are required for reducing the amount of residual monomer, physical properties may be impaired due to deterioration of the polymer. Accordingly, there exists a problem of difficulty in achieving a balance between lowering of residual monomer to a desired level, and keeping or improvement of absorption properties such as absorption capacity without load, absorption capacity under a load and the like. In addition, also in the cases of the above processes (3) and (4), a problem of emission of a bad smell may be raised which is believed to be derived from a component generated upon the heat treatment in the following step through binding of the added reductive substance and impurities derived from the raw material, although detailed grounds are uncertain. Therefore, these processes are not suited for use in production of absorbing articles such as diapers. Moreover, also in the case of the above process (5), when the water absorbing agent was removed from an absorbing article such as a diaper followed by measuring the amount of residual monomer, problems of great variance of the amount of residual monomer of the water absorbing agent among the absorbing articles may be involved in which those having the amount of residual monomer which had not been reduced to a desired level, or those out of spec are found, although the amount of residual monomer is surely reduced. This may result in problems of odor and sanitary aspects, or provide products out of spec as the case may be, under current circumstances in which lowering in thickness of absorbing articles has been preferred, thereby decreasing the amount of pulp or the like of hydrophilic fibers that constitute this absorbing article, and in which further increasing the probability of contact of the water absorbing agent with skin of the user.